The use of overlay programs is not recommended. The information in this appendix is provided for compatibility only. Overlay programs only support load module and PM1. Therefore, any PM format later than PM1 is not supported. Program objects specifying OVLY cause the binder to create either a load module or a PM1 format program object, depending on the library type.

Ordinarily, when a program module produced by the binder is executed, all the control sections of the module remain in virtual storage throughout execution. The length of the module, therefore, is the sum of the lengths of all the control sections. When virtual storage is not at a premium, this is the most efficient way to execute a program. However, when a program approaches the limits of the available virtual storage, you could consider using the overlay facilities of the binder.

In most cases, all that is needed to convert an ordinary program to an overlay program is the addition of control statements to structure the module. You choose the portions of the program that can be overlaid, and the system arranges to load the required portions when needed during execution of the program.

When the binder overlay facility is requested, the program module is structured so that, at execution time, certain control sections are loaded only when referenced. When a reference is made from an executing control section to another, the system determines whether the code required is already in virtual storage. If it is not, the code is loaded dynamically and can overlay an unneeded part of the module already in storage.

This appendix is divided into three sections that describe the design, specification, and special considerations for overlay programs.